Keystone correction circuit



Dec. 1, 1942. o. H. SCHADE KEYSTONE CORRECTION CIRCUIT Filed Dec. 15, 1939 HORJIMPL. HOE. DAMP/N6 70 non. DEFLELTING COILS HORIZONTAL DISCHARGE TUBE SYNC/4. INPUT KEYSTONE CORRECT/0N CONTROL HOR. 5/2 E Z4 CONTROL 6 mm fi RE mam MM m 8 m 1 0 M Lm c u L v1 n m N m E w /M a m Wm m E R WE I NV EN TOR.

0770 H SCHADE A TTORNEY.

Patented bee. i, 1942 PATENT OFFICE KEYSTONE ooamtorionomomr Otto H. Schade, West @aldwell, N. 3., assignor to Radio Corporation of America, a corporation 01' Delaware Application December 15, 1939, Serial No. 309,356

2 Claims.

tical image is projected in a direction normal to the surface of the mosaic electrode. The mosaic of the electrode is scanned by an electron beam on the side on which the image is projected, and in order that the gun structure for producing the electron beam may be out of the optical path, it is necessary that the gun be offset and positioned at an angle with respect to the mosaic electrode. Accordingly, when horizontal and vertical beam deflecting currents or potentials are applied to the deflecting coils or plates, the scanning beam may be caused to scan a rectangular area over a surface or electrode normal to the axis of the gun structure, but when the mosaic electrode is at an angle with respect to the axis of the gun structure, the scanned area then becomes trapezoidal in shape and unless some corrective measures are applied the reproduced image at the receiving tube will also be trapezoidal and will accordingly be distorted as to its actual dimensions. pattern is also formed on a television receiving tube fluorescent screen or target electrode when the electron gun is at an angle other than per pendicular to the target. Heretofore various circuits have been devised for correcting for keystoning effects, but such circuits have generally included one or more additional tubes by means of which the amplitude of the horizontal deflection may be controlled in accordance with the degree of vertical deflection. Such keystone correction circuits, in view of the use ofadditional tubes and circuit arrangements, are subject to loss of adjustment and result in a complication of the deflecting network. The present invention therefore relates to a new and. improved keystone correction circuit wherein no additional tubes are necessary, and in which full keystone correction may be accomplished in a very simple and convenient manner. a

One of the purposes'oi the present invention, therefore, is the provision of the keystone correction circuit in which no additional tubes are A similar trapezoidal necessary over and above the tubes conventionally used ,for causing normal deflection of the cathode ray beam.

Another purpose of the present invention resides in the provision of a keystone correction circuit wherein the degree of correction may be conveniently controlled or adjusted through the manipulation of a single control element or potentiometer.

Still another purpose of the present invention resides in the provision of means whereby the horizontal deflection amplitude of the scanning cathode ray beam may be controlled in accordance withthe vertical deflection of the beam.

A further purpose of the present invention resides in the provision of means whereby the average horizontal deflection of the cathode ray beam may be conveniently adjusted in order to control the size of the rectangular area scanned on the mosaic electrode.

Still a further purpose of the present invention resides. in the provision of a correction network to compensate for any tilting of the scanned area as a result of the non-linear operation of the horizontal deflection amplifier tube.

Other advantages and purposes of the present invention will become more apparent to those skilled in the art through a reading of the following specification and claims, particularly when considered with the accompanying drawing, wherein: v

Figure 1 represents schematically a. television image transmitting tube, the relationship of the used-to accomplish the purposes of the present invention. 7

v Figure 3 shows diagrammatically the areas which are scanned by the'cathode ray beam in the presence and absence of a keystone correction circuit.

Figure 4 shows a curve of the response characteristics of the vertical amplifier tube, and

Figures 5a, 5b and 50 show diagrams of the areas which may be scanned on the mosaic through an adjustment of a corerction'network.

Referring now to the drawing, and particularly to Figure 1 thereof, a television image transmitting tube in is shown. cludes a light responsive mosaic electrode l2, with which is associated a signal plate in the 1 usual manner, known in the art, the output sig- This tube ingun structure l4, by means of which a focused beam of electrons may be produced. Electron beam deflecting means are also used, the horizontal deflecting means being represented by the deflecting coils l6 and the vertical deflecting means being represented by the deflecting coils I8. Obviously, any type deflecting means may be used, and electrostatic deflecting plates may be substituted for the electromagnetic deflecting coils without the exercise of invention.

For focusing an optical image of the object on the light responsive electrode l2, a lens system 22 is provided. It; will be noticed that the light responsive mosaic electrode [2 is arranged normal to the optical axis of the lens system 2 2, and accordingly, when an image of the object 20 is projected on the mosaic electrode l2, no optical distortion takes place provided the lens system is fully corrective. The axis of the gun structure l4 for generating the focused beam of electrons is not normal to the light responsive mosaic surface l2. but is positioned at an angle with respect thereto in order that the gun structure will not obstruct the optical path.

When deflection voltages are applied to the horizontal and vertical deflecting means It and II respectively, a predetermined area of the light responsive electrode 12 will be scanned by the cathode ray beam. If these deflection. voltages are of uniform intensity, then obviouslythe cathode ray beam would scan a rectangular area on a surface normal to the gun structure, but in view of the fact that the light responsive electrode I2 is not normal to the axis of the gun structure, it is clear that a trapezoidal or keystone shaped area on the mosaic electrode will be scanned. In Figure 3 is shown a rectangle A which is the desired area to be scanned on the mosaic electrode, and superimposed thereon is shown by dotted lines a trapezoidal area B which would be scanned by the cathode ray beam if deflection voltages of uniform intensity were applied to the horizontal and vertical deflecting means.

In order to scan a rectangular area on the light responsive electrode of the television image transmitting tube, it is necessary that the amplitude of the horizontal deflection of the oathode ray beam be varied in accordance with the vertical displacement of the beam, since the lower edge of the mosaic electrode is nearer the gun structure than the upper edge. Therefore, when the cathode ray beam is scanning the lower .edge of the mosaic electrode it is necessary that the horizontal deflection be increased a certain amount over the deflection which is necessary when the cathode ray beam is scanning the upper edge of the light responsive electrode.

This keystone correction is accomplished in accordance with the present invention by the circult shown in Figure 2. This circuit comprises an electron discharge tube 24 which includes a cathode, a control electrode and an anode 25. The cathode of the tube 24 is connected to ground and synchronizing or driving impulses are applied between the control electrode'and the cathode by way of the terminals 26. The anode 25 is maintained positive with respect to the cathode of the tube 24 by means of a source of positive potential which is connected to the terminal 28. The anode 25 is connected to the positive terminal 28 by means of a load resistasoaoov ance 36; a portion of potentiometer 32 and a portion of, potentiometer 34. 'The anode 26 is also connected to ground by way of condenser 36,

which operates as the charging condenser for producing the desired voltage variations which ultimately cause horizontal deflection of the cathode ray beam.

Normally, the horizontaldischarge tube 24 is rendered non-conductive due to properly biasing the potential of the control electrode thereof,

and during this non-conductive period the con- 4 cation of a synchronizing or driving impulse to this tube, the tube is rendered conductive and the charge which has accumulated on condenser 36 is substantially instantaneously removed by conduction through the tube. After the removal of the synchronizing or driving impulse, the control electrode of tube 24 again biases the tube to cut-oif and the condenser 36 again gradually assumes a charged condition. This operation is continued so long as driving impulses are applied to the control electrode of the tube, and accordingly, voltage variations of saw-tooth wave form may be derived from the anode 25 of the horizontal oscillator discharge tube 24.

Since these voltage variations of saw-tooth wave form as produced directly by the horizontal discharge tube 24 are not of sufficient intensity to cause the necessary horizontal deflection amplitude of the cathode ray beam, a horizontal deflection power amplifier tube 38 is included. Since horizontal deflecting currents of considerable intensity are generally necessary in the deflecting coils, the horizontal amplifier tube 38 is generally of the beam power type and includes a cathode, a control electrode, a screen electrode and an anode. The control electrode of the horizontal amplifier tube 38 is connected to ground by means of the grid resistor 40, and in order that the desired grid bias may be maintained on the tube, the cathode of the tube is connected to ground by way of resistance 42 which is by-passed by condenser 44. The screen electrode is connected to the positive terminal 28 and the anode includes the primary of the output coupling transformer 46. The voltage variations of saw-tooth wave form which appear at the anode 25 of the horizontal discharge tube 24 are applied to the control electrode of the horizontal amplifier tube 38 by way of the coupling condenser 48. Connected between the coupling condenser 48 and the anode 25, is a correction network including resistance 50 in parallel with which is connected a small condenser 52. The purpose of this resistance and condenser will be explained later. When the voltage variations which are produced by the horizontal discharge tube 24 are applied to and amplified by the horizontal amplifier 38, consider-- In order to produce the appropriate horizontal deflection and to reduce the electron beam return time as well as to improve the linearity of such deflection, a horizontal damping tube 54 is used which includes a cathode and an anode. The cathode of the damping tube is connected directly to the anode of the horizontal amplifier tube 38, while the anode of the damping tube 56 is connected to the positive terminal 28 by way of resistance 58 and the parallel condenser Ell. The purpose and operation of the horizontal damping tube 56, as well as its associated resistance 58 and condenser 60, is well known to those skilled in the art.

One end of the potentiometer 32 is connected to the movable contact of the potentiometer 34 while the other end of the potentiometer 32 is connected to ground by way of resistance 62. The load resistance 30 of the horizontal oscillater 34 is connected to the movable contact of the tentiometer 32 and in view of this connection the average direct current potential which appears at the anode of the discharge tube 24 and at condenser 36 may be varied in accordance with the position of the movable contact along the potentiometer 32. Accordingly, a variation in the position of the movable contact along the potentiometer 32 will operate to vary the amplitude of the average horizontal deflection of the cathode ray beam in the television transmitting tube, since a variation in the position of the movable contact of potentiometer 32 alters the potential which is available to charge the condenser 36. Accordingly the amplitude of the saw-tooth voltage variations which are derived from the anode of tube 24 may be controlled in accordance with the setting of the potentiometer 32.

The oscillator for producing the vertical deflection voltage variations is not shown in the drawing since this element may be in the form of any desired oscillator such as a multivibrator relaxation oscillator, or blocking oscillator. The output from such oscillator is, however, applied to the vertical deflecting voltage amplifier tube 34 which includes a cathode, a control electrode and an anode 55. The voltage variations from the vertical oscillator are impressed between the control electrode and the cathode of amplifier tube64 by application to the terminals 66. In order to properly bias the control electrode of the amplifier tube 34 a cathode potentiometer 33 is provided, the used portion of which is bypassed by means of condenser ill. The movable contact of potentiometer 68 is connected to ground in order that an adjustment of the potentiometer 53 may vary the amount of the cathode resist- .ance included in the circuit of tube 64. An adjustment of this potentiometer varies the linearity of the vertical deflecting voltages in a manner well known to those skilled in the art.

The anode of tube 35 is maintained positive with respect to its cathode by connecting the anode to the positive terminal 28 by way of resistance l2 and the primary of the transformer 714. This circuit also includes resistance 16 which is used for decoupling purposes, and the junction of the primary of transformer 14 and resistance 43 is connected to the cathode of the amplifier tube 64 by means of a by-pass condenser 13. The secondary of transformer '44 is connected to the terminals 83 and furnishes the necessary deflection current. These terminals 80 are therefore accordingly connected to the vertical deflecting means 43 of the television image transmitting tube. The anode of tube 65 is also connected to the positive potential terminal 28 by way ofthe potentiometer 34 which is preferably of relatively high resistance. The movablecontact of this potentiometer as explained above, is connected to one end of the potentiometer resistance 32.

Since the amplifier tube 34 operates to amplify the vertical deflecting voltages, the amount of current drawn by the tube 64 will vary cyclically in accordance with the vertical deflecting oscillations. At the anode end of the potentiometer 34 the entire voltage variationof the anode 65 will, therefore, be available, whereas the othe end of potentiometer 34 remains fixed in potential since it is connected directly to the positive terminal 38. The movable contact of the potentiometer 34, therefore, will be subjected to voltage variation of vertical deflection frequency, the amplitude of the voltage variation being determined in accordance with the position of the movable contact along the potentiometer 34. Since this potentiometer, or at least a portion thereof, is included in the anode circuit of the horizontal discharge tube 24, the voltage which is available to cause the condenser 36 to be charged, will be cyclically varied (or modulated in intensity) at a frequency equal to the vertical deflection frequency. As explained above, the intensity of this voltage variation of vertical deflection frequency depends upon the position of the movable contact along the potentiometer 34. In view of this connection, and the inclusion of the potentiometer 34, the condenser 36 which is cyclically charged and discharged at horizontal deflection frequency, will be modulated at vertical deflection frequency and accordingly, the voltage variations of saw-tooth wave form and of horizontal deflection frequency which are available from the anode 25 of tube 24 will be modulated by a certain amount at vertical beam deflection frequency. Sincethe 'amplitude of the horizontal deflecting voltage as derived from the terminals 54 depends upon the intensity of the voltage variations as applied to the control electrode of tube 38, then, accordingly, the horizontal deflection currentswhich are available-at the terminals 54 and which are applied to the horizontal deflecting means l6,

will be modulated in intensity at a rate determined by the vertical deflection frequency and in an amount determined by the position of the movable contact along the potentiometer 34.

From the above it may be seen, therefore, that the inclusion ofthe potentiometer 34 and its connection to the circuit as described above, will cause a modulation of the horizontal deflection voltage and accordingly, a keystone correction will result, the amount of correction being adjustable in accordance with the adjustment of the movable contact along the potentiometer 34. When the potentiometer 34 is properly adjusted, it is obvious that a true rectangular area may be scanned on the light responsive electrode l2 even though this electrode is not normal to the axis of the electron gun structure.

Since the modulation voltage applied to the horizontal amplifier tube 38 is not exactly inherently linear, some means must be provided for compensating for such lack of linearity. In Figure 4 is shown a curve 32 of th modulating volt- .age as derived from the vertical amplifier tube 34. Ideally, the modulating voltage curve 82 should be linear as represented by the dotted line 84. If no means is provided for correcting for this lack of linearity, a pattern such as shown at 85 in Figure 50 will be traced on the light responsive electrode I2 of the television transmitting tube 10. When the resistance 50 and condenser 52 are included in the anode-cathode circuit of tubes 24 and 3B, the wave form of the vertical saws-tooth voltage that'is impressed upon the control electrode of tube 38 from the vertical amplifier 64 is altered so as to give linear distribution of the scanning lines on the mosaic electrode. This filter section, including resistance 50 and condenser 52, is adjusted so that a true rectangular pattern such as indicated at 81 in Figure So will be traced on the light responsive electrode and a variation in valued the resistance 50 is ordinarily sufficient to accomplish the desired results. Ifthe resistance 50 is too large then over-1 compensation results and a pattern such as indicated at 86 in Figure 5b will be scanned onthe light responsive electrode. The condenser 52 by itself has a reactance sufliciently high to eliminate coupling of vertical modulating frequency into the control grid of, the horizontal amplifying tube 38, but is sufflciently low to transmit the higher frequency horizontal sawtooth voltage to tube 38.

The resistance 50 will, therefore, permit coupling of the modulating voltage from the vertical amplifying tube Bl (curve 82) into tube 38, thus permitting correction of the modulation envelope curve so as to avoid scanning a distorted pattern such as shown in Figures 5a or 5b. The adjustasoaosv put and an output electrode, means including said discharge tube for generating voltage variations of substantially sawtooth wave form of a relatively high fre uency, a source of voltage variament of the resistance 50 is to a certain extent dependent upon the position of the movable contact along the potentiometer 34, but in view of the uniformity in television image transmitting tubes, after the potentiometer 3| and the resistance 50 are initially set no further adjustment of necesdevised by means of which full and complete keystone correction may be developed, and by means of which'a rectangular area may be scanned on the mosaic electrode of a television transmitting tube, the rectangular area having the desired proportions or aspect ratio. v

. While the invention is described with particular reference to television tubes of the transmitting type, it will be obvious that the invention is equally as. applicable for keystone correction in the use of tubes of the receiving type wherein the fluorescent screen or target is positioned at an angle to the electron gun so that the targetmay be viewed from the scanned side.-

While it is conceivable that various alteratio or modificationsmay be made inthe present invention without departing from the spirit and scope thereof, it is desired that any and all such modifications be considered within the purview of tions of substan ially sawtooth wave form-of relatively low frequency, means for impressing a predeterminedportion of the voltage variations of the said relatively low frequency upon the output electrode of the discharge tube whereby the voltage variations of the said relatively high frequency will be modulated in amplitude by the said relatively low frequency, an amplifier tube having input and output electrodes, a coupling circuit including a parallel connected resistance'and condenser, and means including a condenser .for connecting the coupling circuit between the' output electrode of the discharge tube and the input electrode of the amplifier tube whereby the condenser of the coupling circuit will pass the high frequency component of the modulated voltage variations and the resistance of the coupling circuit will pass the low frequency component of the modulated voltage variations in order that the linearity of the voltage variations may be improved.

2. A system of the character described for use in a television system comprising a source of voltage variations of relatively low frequency and of sawtooth wave form, an electron discharge tube having a cathode, a control electrode, and an anode, a condenser connected between said anode and said cathode, means for impressing control impulses of a relatively high frequency between the control electrode and the cathode of said discharge tube. means modulated by the source of low frequency voltage variations for maintaining said anodepositive with respect to said cathode, whereby voltage variations of said relatively high frequency may be present atthe anode of said discharge tube, the voltage variations being amplitude modulated at said relatively low frequency, an amplifier tube for amplifying the modulated high frequency voltage variationscomprising a to said cathode. a coupling circuit including a condenser and a resistanc connected in parallel,

means for connecting one end of the coupling circuit to the anode of said discharge tube, and means including a condenser for connecting the other end of the coupling circuit to the control electrode of theiimplifier tube whereby the condenser of the coupling circuit will pass the high frequency voltage xariations and the resistance of the coupling circuit will pass the low frequency modulating voltage variations in order that the linearity of the composite voltage variations may beimproved.

OI'IO H. SCHADE. 

